<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <meta http-equiv="X-UA-Compatible" content="ie=edge" />
    <title>Document</title>
    <style>
      body {
        margin: 0;
        padding: 0;
        background: linear-gradient(to bottom, #000000 0%, #5788fe 100%);
      }

      canvas {
        width: 100%;
        height: 100%;
      }
    </style>
  </head>

  <body>
    <canvas></canvas>
    <script>
      //获取canvas元素,并给canvas设置大小
      var can = document.querySelector("canvas");
      can.width = window.innerWidth;
      can.height = window.innerHeight;
      //canvas大小随浏览器一起变化
      window.onresize = function() {
        can.width = window.innerWidth;
        can.height = window.innerHeight;
      };
      //获取2D绘图环境
      var ctx = can.getContext("2d");
      //返回一个min和max之间的一个随机整数
      function randow(min, max) {
        return Math.round(Math.random() * (max - min) + min);
      }
      //返回一个min和max之间的一个随机小数
      function random(min, max) {
        return Math.random() * (max - min) + min;
      }

      //用来存放每个圆的属性
      var dots = [];
      //生成圆的个数
      var arcNum = 100;
      // 存储n个不同属性的圆
      function createArc() {
        for (var i = 0; i < arcNum; i++) {
          //定义一个对象，将圆的属性存储起来
          var arcObj = {
            arcX: randow(0, window.innerWidth), //圆的X坐标
            arcY: randow(0, window.innerHeight), //圆的Y坐标
            arcR: 2, //圆的半径
            color: `rgba(${randow(0, 255)},${randow(0, 255)},${randow(
              0,
              255
            )},${random(0, 1)})`,
            suduX: random(-0.5, 0.5), //圆X轴速度
            suduY: random(-0.5, 0.5) //圆Y轴速度
          };
          dots.push(arcObj);
        }
      }
      createArc();
      // 画圆
      function draw() {
        ctx.clearRect(0, 0, window.innerWidth, window.innerHeight);
        for (var i = 0; i < arcNum; i++) {
          //让圆动起来
          dots[i].arcX += dots[i].suduX;
          dots[i].arcY += dots[i].suduY;
          ctx.beginPath();
          ctx.fillStyle = dots[i].color;
          ctx.arc(dots[i].arcX, dots[i].arcY, dots[i].arcR, 0, Math.PI * 2);
          ctx.fill();
          ctx.closePath();
          //边界检测,将速度取反,实现碰撞
          if (dots[i].arcX <= 0 || dots[i].arcX > window.innerWidth)
            dots[i].suduX *= -1;

          if (dots[i].arcY <= 0 || dots[i].arcY > window.innerHeight)
            dots[i].suduY *= -1;

          //利用勾股定理判断是否连线 a*a+b*b=c*c
          //  Math.sqrt() 平方根
          //  Math.pow(a,b)  a的b次方
          for (var j = i + 1; j < arcNum; j++) {
            if (
              Math.sqrt(
                Math.pow(dots[i].arcX - dots[j].arcX, 2) +
                  Math.pow(dots[i].arcY - dots[j].arcY, 2)
              ) < 100
            ) {
              ctx.beginPath();
              ctx.strokeStyle = dots[i].color;
              ctx.moveTo(dots[i].arcX, dots[i].arcY);
              ctx.lineTo(dots[j].arcX, dots[j].arcY);
              ctx.closePath();
              ctx.stroke();
            }
          }
        }
      }
      setInterval(draw, 60 / 1000);
      //添加鼠标移动事件
      can.onmousemove = function(e) {
        var ev = event || e;
        var mouseX = ev.offsetX;
        var mouseY = ev.offsetY;
        for (var i = 0; i < arcNum; i++) {
          if (
            Math.sqrt(
              Math.pow(dots[i].arcX - mouseX, 2) +
                Math.pow(dots[i].arcY - mouseY, 2)
            ) < 100
          ) {
            ctx.beginPath();
            ctx.strokeStyle = dots[i].color;
            ctx.moveTo(mouseX, mouseY);
            ctx.lineTo(dots[i].arcX, dots[i].arcY);
            ctx.closePath();
            ctx.stroke();
          }
        }
      };
    </script>
  </body>
</html>
